What makes a "Song of the summer"? For most, it's the association with a memory: a track discovered on a drive to the shore, perhaps, or the biggest hit at summer camp. It usually has some sort of carefree nature, an aural shot of sunshine popping out of car speakers and, today, laptops and smartphones.

While that sounds very personal, Billboard has a different definition. The music industry publication and keeper of the airplay and sales charts names an official Song of the Summer based on its rankings each year, and has retroactively awarded songs all the way to the summer of 1985 (when "Shout" by Tears For Fears ruled radios).

Can those past winners tell us anything about 2015's crop of contenders? amNewYork took a look at the chart positions and characteristics of the Song of the Summer for the past 15 years to see if there's a formula for the award, and while exceptions certainly exist, we did find some patterns:

1) You've already heard it ...No Song of the Summer since 2004's "Confessions Part II" by Usher has actually been released as a single during the summer. Even in that case, the song entered the Hot 100 on May 1 -- before its official release.

2) ... but it's still in its ascendancyNo winner out of the last 15 got to the top of the Billboard chart before June, meaning that a song like "Uptown Funk" by Bruno Mars and Mark Ronson likely peaked too soon -- even with its staying power.

3) By Memorial Day, it's inescapableThree of the last five winners were in the top five of the Hot 100 by the end of May. Of the last five, only "Blurred Lines" charted lower than 20 on holiday weekend.

4) It has to hit the top of the Billboard Hot 100 -- and it should do so before the end of JuneEve and Gwen Stefani's "Let Me Blow Your Mind" was the Song of the Summer of 2001, despite never hitting the top of the Hot 100. Since then, seven of the 13 winners hit the No. 1 spot before the end of June, including four of the last five (LMFAO's "Party Rock Anthem" was the lone holdout, in 2011)

5) It'll be fastSeven of the last nine winners had a beats-per-minute count of more than 100, and six of those were more than 120. It's bad news for Wiz Khalifa's "See You Again" which has been hovering around the top of the charts.

6) It'll be (relatively) cleanSorry, Rihanna's "[Expletive] Better Have My Money.". So, what songs does that leave?. The only songs to make it to the top of the Hot 100 thus far in June of 2015 are "See You Again" and Taylor Swift's "Bad Blood." Both are slower than many recent winners, though America's love for T-Swift has little care for rules.

Fetty Wap's "Trap Queen" has already been on the chart for four months, but can't seem to break through to the No. 1 spot. Other contenders like Jason Derulo's "Want to Want Me" and the David Guetta/Nicki Minaj collaboration "Hey Mama" feel more like summer smashes, but can't seem to break into the public consciousness.